The present invention relates generally to a method for improving the adhesion between two polymer layers or films. In particular, this invention relates to a method in which a silicon oxide-containing layer is formed on the surface of a polyimide layer, and a second layer of polymeric material is formed on the silicon oxide-containing layer.
Polyimides have physical properties that make them especially suited for microelectronic applications. These properties include high thermal stability, low dielectric constant, excellent adhesion to substrates, high radiation stability, excellent planarization, and high flexibility relative to inorganic dielectrics. Thus, polyimide layers are widely used in the electronics industry to provide protection, electrical insulation, or both. They are used, for example, as protective coatings for semiconductors, as dielectric layers for multilayer integrated circuits, as high-temperature solder masks, as bonding for multilayer circuits, and as the final passivation coating on electronic devices.
Polyimide layers may be formed by vapor deposition techniques in which a precursor is deposited on the substrate and then thermally cured. Vapor deposition techniques are slow and impractical, however, when thick layers (greater than 500 nm) are desired. Alternatively, polyimide layers may be formed by cross-linking solution-deposited polyimide precursor. The total thickness that can practically attained in a single layer is limited, however, to about 25 microns. If a thicker layer is desired, it is necessary to apply a second layer of polyimide precursor on top of the first polyimide layer. In addition, in some applications, it is necessary to form a second polyimide layer, or a layer of a different polymer, on a previously applied polyimide layer to provide protection or insulation.
Poor adhesion results at the interface between the first polyimide layer and the second layer of polymer. To improve the adhesion between the layers, it is necessary to treat the surface of the first layer before the second layer is deposited. Treatment typically requires wet chemical processing. One such method, for example, involves the hydrolysis of cured polyimide film with an alkaline solution in a solvent or a mixture of solvents to produce a polyamic acid salt, which is transformed to a polyamic acid-containing surface onto which a liquid polyimide precursor is deposited and subsequently cured. See, for example, L. J. Matienzo and W. N. Unertl, xe2x80x9cAdhesion of Metal Films To Polyimide,xe2x80x9d in Polyimides: Fundamentals and Applications, Chapter 21, p. 659 (M. K. Gosh and K. L. Mittal, eds., Marcel-Dekker, New York, 1996); and N. C. Stoffel, xe2x80x9cInterdiffusion and Adhesion of Polyimides,xe2x80x9d Ph.D. Thesis, Cornell University (1995).
Thus, a need exists for a method for improving the adhesion between a polyimide layer and a second layer of polymer, especially a second polyimide layer, that does not require wet chemical processing.
To meet this and other needs, and in view of its purposes, the present invention provides a method for improving the adhesion between a first polyimide layer and a second layer, especially a second polyimide layer, that does not require wet chemical processing. The method comprises:
(a) forming a doped layer over a substrate, the doped layer comprising (1) an organo-silicon compound and (2) either a first polymeric material or a first precursor composition that can be converted to a polymeric material after the doped layer has been applied, the doped layer having an outer surface and an inner surface, the inner surface facing the substrate;
(b) heating the doped layer and forming an organo-silicon-rich layer on the outer surface of the doped layer;
(c) converting the organo-silicon-rich layer to a silicon oxide-containing layer; and
(d) forming an added layer over the organo-silicon-rich layer, the added layer comprising either a second polymeric material or a second precursor composition that can be converted to a polymeric material after the added layer has been applied to the silicon oxide-containing layer.
In one embodiment, the doped layer comprises a polyimide precursor composition. In another embodiment, the added layer comprises a polyimide precursor composition that, preferably, is converted to a polyimide layer. In still another embodiment, a layer of adhesion promoter is applied to the substrate before step (a). In a further embodiment, the invention is a composite structure comprising a substrate, a polymeric layer, a silicon oxide-containing layer, and an added layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.